Case fans powered off 6.3V heaters

Hey guys, I'm trying to power 2 case fans off the 6.3vdc heater rail in my dual gates sta build but I'm getting a lot of hum through the audio when the fans are connected. What would be the best way to connect them up? I'm trying to avoid buying a 240v fan and running it off mains, or an external PSU for them, but might have to if there's no way to get them to stop interfering.

Thanks!

George

To be honest you would be better just having a really small psu inside the unit just for the fans, consumption is low, it would be a really small board.

are those 12v fans?

Maybe look at some power amplifier schematics?

I know the old Altec 9444B series had a fan..... for example.... QSC,Crown,.....

This is a wild guess but try isolating by converting the 6.3VAC to DC and use a DC fan. But you might need some series resistance immediately after the bridge rectifier to prevent rectifier noise from bleeding backward. So rectifier -> 2 resistors -> filter cap -> fan. And try with and without tying one side to ground with a 10k resistor. You might need a 5V fan though. Try a USB fan if you have one just to see if it works. Adjust the two resistors to control the speed vs noise reduction ratio.

Actually did you try just adding two resistors in series with each input to the AC fan? They would have to have an appropriate power value of course. Yeah, the fan will loose more or less power depending on the size of the resistors but it might just be enough to snub the noise.

The noise is probably because the current draw of the fan is not smooth. A resistor might smooth it out just enough.

Also, just like heater wires, it's important that the feed wire and return wire are dressed close to one another. Meaning gently twisted together.

Also, try physically moving the fan away from tubes as far as possible just to see if it's the actual fan itself putting out EMI that is getting picked up by high impedance circuitry. Use a temporary cable extension if you have to. If the fan is 0.25 meters away from any sensitive electronics and the hum goes away, then the problem is the fan itself and not the supply to it in which case there may be nothing you can do except move the fan physically far away.

Thanks for the suggestions. The fans are actually 5v usb fans and the heaters are on DC already. I’m using a resistor to drop the voltage to 5v and the noise doesn’t seem to change when I move the fans away from the compressor. Would a cap across the fan help me at all? I’m not sure how to find the correct value if that was the case.

well a 5 Volt PSu for the fans only is something that is so cheap that is almost for free, no need whatsover to take the fans output from the heater supply.

Isolate them

Great point. Decided to just go with a separate supply. Nice and neat Thanks for the help!

Just for the record, might help future users: I often add fans to guitar amplifiers.

I bridge rectify 6.3V AC from filaments, use a2200/ 4700uF x 16V capacitor and use resulting 8 VDC to feed plain 12V DC PC supply fans.

They work fine at the reduced voltage, move less air which is no problem, I am not blowing hard into an SS amplifier heat sink which needs it but simply renovating hot air inside a typical tube amp chassis or cabinet, way better than "natural ventilation"  if you ask me.

Bonus? : whisper quiet.

No added hum or buzz, feed line to fan is NOT grounded but floating ... as floating as filaments by the way.

Just be careful not to put reverb tank pickup head near fan or it will easily pickup its crazy magnetic flux waveform.